Code review comment for lp:~al-maisan/launchpad/qualify-subquery-507562

Hello there!

The selection of candidate jobs for idle builders in the build farm uses a
general query part that may select too many candidates.
Thus, all build farm job type classes get a chance to add sub-queries that
further refine or narrow down the set of selected candidate jobs.

However, these sub-queries need to be put into context so that they apply only
to the jobs beloging to the job class that contributed the sub-query.

That is what the branch at hand does.

Please note that all _findBuildCandidate() tests (we have a pretty extensive
test coverage of the build farm candidate job selection) pass.

Test to run:

    bin/test -vv -t build

No "make lint" errors or warnings.